The invention concerns a process for manufacturing a capacitor dielectric with inner blocking layers made of polycrystalline ceramic solids of a material with a perovskite structure on the basis of barium titanate of the general formula EQU (Ba.sub.1-x M.sub.x.sup.II) O . z(Ti.sub.1-y M.sub.y.sup.IV)O.sub.3
with M.sup.II = Ca, Sr, Pb and/or Mg and M.sup.IV = Zr, Sn, whereby z encompasses the values from 1.005 to 1.05. The dielectric contains at least two different doping substances, of which one (Antimony, Niobium, Lanthanum or Bismuth) causes in the interior of the crystallite predominantly n-type conductivity and the other (Copper, Iron, Cobalt, or Manganese) causes in the surface layer of the crystallite predominantly p-type conductivity. The proportion of the doping substance which causes the n-type conductivity is larger by a factor of 1.5 to 2.5 than the maximum doping amount. The proportion of the substance which causes the p-type doping amounts to 0.01 to 0.15 percentage by weight.
In the process, the components which are necessary for the manufacture of the bodies are mixed in oxide form or in a form which produces oxides, are ground wet or dry, and are afterwards brought up to solid state reaction at temperatures between 950.degree. C. to 1100.degree. C., after which the product of the reaction is ground again until the desired particle size is reached. The solids are produced out of the powder by means of pressing and are then subjected to sintering at 1250.degree. C. to 1450.degree. C. for 1 to 6 hours.
Such a process for the manufacture of this capacitor dielectric is described in German Auslegesschrift No. 1,614,605 or in corresponding British patent GB-PS No. 1,204,436 or U.S. Pat. No. 3,569,802.
One capacitor dielectric which is cited in these documents has been on the market for several years under the name SIBATIT 50,000 (SIBATIT is a registered trademark) and has been technologically tested many times. This capacitor dielectric can be used in the form of disks, tubes with circular and with square-shaped cross sections, whereby the common metal layers (e.g., silver) are always used as coatings. The dielectric can also be used in the form of so-called stacking capacitors. Stacking capacitors are the type whose thin layers of dielectric material are arranged one on top of the other with metal layers which protrude to the edge of various sides in alternating fashion. The metal layers are subjected to sintering in this stacked arrangement.
In order to cause maximum conductivity in the interior of the granule with the simultaneous presence of the p-doping substance, despite the proportions of n-doping substance which are higher than those which are normally necessary for the maximum conductivity (maximal doping), the cited documents suggest bringing all the substances together to a reaction in oxide form. In this case the conductivity in the interior of the crystalline granules reaches the highest possible values, whereas the doping substance, in particular copper, which either cannot be incorporated or incorporated only to a limited extent into the perovskite grid, is incorporated essentially into the surface layer of the crystalite.
According to the known process a powder is produced therewith after the solid state reaction of the starting materials. The dielectric solids are immediately produced out of this powder, e.g., through pressing by means of an extrusion press or by means of a standing press, whereby the pn-transitions in the finished solid, which are formed onto the crystallite granules in the surface areas, then become dielectrically effective when voltage is applied.
When one refers to values for the dielectrically constant (DK) in the case of a capacitor dielectric with interior blocking layers, then the apparent DK values are always referred to since in establishing the DK from the measurement of the capacity of such a capacitor it is assumed that the total solid has a high .epsilon., whereas in fact only the very thin pn-transitions become dielectrically effective at the granular granule boundaries. These exhibit the usual DK value for barium titanate. However, due to the relation of the total solid, the dielectric has a DK which is raised by many times, for example 30,000 to 100,000, whereas BaTiO.sub.3 itself has only DK values of 1000 to 3000 normally.
In a capacitor dielectric not only does the DK play a role in view of the size of the capacity, but it is also necessary that the dependence of the DK on an operating temperature, the tangent of the loss angle (loss factor), and the insulation and thereby the capacitance of the capacitor are located within certain boundaries in the case of higher field intensities.
This is already the case to a large extent for the electrical properties which have been listed here with respect to the capacitor dielectric which was cited above and which has interior blocking layers.
Despite that, it is desirable to bring closer together the limits of tolerance which in part lie relatively far apart, i.e., to make the tolerances of the electrical values which are determined by manufacture smaller and thereby to achieve each time in mass production even more precisely the reproducibility of the desired electrical values. Limits which are too broad for the electrical values occur especially when rutile in the form of TiO.sub.2 components is used for the production of perovskite forming materials. This results because then, on the one hand, the DK values are relatively high (up to 100,000) and, on the other hand, the variance of these DK values from capacitor to capacitor is relatively great for the same baking charge with differing sintering conditions (e.g., different sinter ovens).